Glucocorticosteroids (GC) induce lymphocytolysis, a form of programmed cell death (PCD) or apoptosis, and are therefore among the most effective agents for treating lymphoid malignancies. This competing renewal application will test and refine a PCD model based on emerging evidence that depletion of an endoplasmic reticulum (ER) Ca2+ pool critical for cell growth and survival is a central, bcl-2-regulated step in the lymphocyte PCD pathway. The first aim will investigate how bcl-2 and bcl-X1 inhibit PCD by testing two hypotheses: (i) that prevention of ER Ca2+ pool depletion by bcl-2 and bcl-X1 inhibits apoptosis; and (ii) that bcl-2 and bcl-X1 prevent ER Ca2+ pool depletion by regulating ion channels in the ER membrane. The second aim will investigate what governs whether a cell recovers from ER Ca2+ pool depletion or undergoes apoptosis by testing two hypotheses: (i) that increased transcription of 'protective response genes; enables a cell to recover normal growth and maintain viability in the presence of a decreased ER Ca2+ pool; and, (ii) that GC repress transcription of 'protective response genes', preventing recovery of cell growth and leading to cell death in response to ER Ca2+ pool depletion. The third aim will investigate how sustained ER Ca2+ pool depletion induces apoptosis by testing two hypotheses: (i) that a constitutively expressed endonuclease is confined to the ER until it is released into the nucleus in response to sustained ER Ca2+ pool depletion; and, (ii) that sustained ER Ca2+ pool depletion regulates transcription of 'suicide gene(s)'. In summary, the proposed research will provide novel insight into molecular mechanisms of PCD in lymphocytes by testing and refining a model that already has strong support based on work in this laboratory during the previous funding period.